Thanks, Chuck!   I'll give this a try.

How can I use a UI reference variable to only show one column? so that the search occurs in that column

Use something like below. It will show in the UI Page.

<g:ui_reference label="Name" name="name" table="sys_user" field="user_name" />

How are you using the UI Page on a Record Producer.   As a popup with GlideDialogWindow or something?

I think what you need is a Variable on your Record Producer (you have hide it with a UI Policy). The value from the UI Page needs to be copied to the Variable with a Client Script. When you submit the Record Producer, you can map the Variable to the Name field on your new record.

I have cross posted this question to: Live 4 hr Q&A session with Service Catalog Product Experts

Hopefully we'll have an answer today during the session.